Room: Room 202
Purpose: The internal organ motion is clinically assessed using 4DCT during radiotherapy treatment planning with the presumption of good respiratory-induced organ motion reproducibility. However, this presumption can often be violated by the irregular respiratory pattern as revealed by other surrogate or imaging studies. We aimed to evaluate the inter-(respiratory)-cycle and inter-subject reproducibility of the kidney motion using a fast volumetric 4D-MRI
Methods: Abdominal 4DMRI were axially acquired on 9 healthy subjects (34Â±6 years) using a CAIPIRINHA â€“accelerated 3D gradient echo VIBE sequence (voxel size = 2.7x2.7x4mm3, 56 slices, 1.63 volume/second, 144 volumes, free-breathing). Left(LK) and right kidneys(RK) were manually segmented on the chronologically 2nd volume (reference volume). The remaining volumes were rigidly registered to the reference volume to obtain the time-resolved volumetric kidney motion along superior-inferior(SI), anterior-posterior(AP) and left-right(LR) directions. For both kidneys, the motion range (âˆ†r) was calculated in each respiratory cycle. The inter-subject reproducibility of kidney motion was evaluated with the coefficient of variation(CV) of the motion range of all subjects. For each subject, the inter-cycle variability was calculated by the ratio of the standard deviation of the motion range difference between respiratory cycles to the mean motion range.
Results: The smaller CVs (better inter-subject reproducibility) were obtained along SI (LK=0.26(CV), 10.0Â±2.7mm(âˆ†r); RK=0.27(CV), 10.4Â±2.8mm(âˆ†r)) than that along AP (LK=0.58(CV), 2.3Â±1.3mm(âˆ†r); RK=0.54(CV), 4.2Â±2.2mm(âˆ†r)). CV along LR was not reported because it had very small âˆ†r and, thus, was more susceptible to sources of uncertainty. The smallest inter-cycle reproducibility was observed along SI for both kidneys (LK=0.20Â±0.07; RK=0.18Â±0.06). Larger inter-cycle variability was observed in LK than RK in all directions (LK=0.20Â±0.07(SI), 0.48Â±0.43(AP); RK=0.18Â±0.06(SI), 0.27Â±0.10(AP)).
Conclusion: The inter-subject and inter-cycle reproducibility of the renal motion was <0.27 and <0.20, respectively for both kidney along SI direction. Our results might be useful in evaluating the treatment delivery efficiency in motion compensated radiotherapy.